load average

\linux\include\linux\sched.h中定义的有关频率，load average的相关常数与变量

#define HZ 100  //定义时钟频率，两个时钟间隔时间为10ms
/*
 * These are the constant used to fake the fixed-point load-average
 * counting. Some notes:                                                                       *  - 11 bit fractions expand to 22 bits by the multiplies: this gives 11bit
 *    a load-average precision of 10 bits integer + 11 bits fractional
 *  - if you want to count load-averages more often, you need more
 *    precision, or rounding will get you. With 2-second counting freq,
 *    the EXP_n values would be 1981, 2034 and 2043 if still using only
 *    11 bit fractions.
 */
extern unsigned long avenrun[];		/* Load averages */

#define FSHIFT		11		/* nr of bits of precision */
#define FIXED_1		(1<<FSHIFT)	/* 1.0 as fixed-point */
#define LOAD_FREQ	(5*HZ)		/* 5 sec intervals */
#define EXP_1		1884		/* 1/exp(5sec/1min) as fixed-point */
#define EXP_5		2014		/* 1/exp(5sec/5min) */
#define EXP_15		2037		/* 1/exp(5sec/15min) */
#define CALC_LOAD(load,exp,n) \    //n=active_tasks活动进程数 ，load各个exp计算得到的average load
	load *= exp; \
	load += n*(FIXED_1-exp); \
	load >>= FSHIFT; 

load=(exp*load+n*(2048-exp))>>11。由这个式子来看，所谓的load只与当前活跃进程数有关。

                                                                     \linux-2.4.x\kernel\timer.c中定义load average的计算函数。
unsigned long avenrun[3];

static inline void calc_load(unsigned long ticks)
{
	unsigned long active_tasks; /* fixed-point */
	static int count = LOAD_FREQ;
 
	count -= ticks;
	if (count < 0) {
		count += LOAD_FREQ;
		active_tasks = count_active_tasks();
		CALC_LOAD(avenrun[0], EXP_1, active_tasks);
		CALC_LOAD(avenrun[1], EXP_5, active_tasks);
		CALC_LOAD(avenrun[2], EXP_15, active_tasks);
	}
}

\linux-2.4.x\fs\proc\proc_misc.c中定义使用uptime命令时调用load average显示函数的实现方式

static int loadavg_read_proc(char *page, char **start, off_t off,
     int count, int *eof, void *data)
{
 int a, b, c;
 int len;

 a = avenrun[0] + (FIXED_1/200);
 b = avenrun[1] + (FIXED_1/200);
 c = avenrun[2] + (FIXED_1/200);
 len = sprintf(page,"%d.%02d %d.%02d %d.%02d %d/%d %d\n",
  LOAD_INT(a), LOAD_FRAC(a),
  LOAD_INT(b), LOAD_FRAC(b),
  LOAD_INT(c), LOAD_FRAC(c),
  nr_running, nr_threads, last_pid);
 return proc_calc_metrics(page, start, off, count, eof, len);
}

 http://www.yeeyan.org/articles/view/79184/37314